The Neogrammarians (also Young Grammarians, German Junggrammatiker) were a German school of linguists, originally at the University of Leipzig, in the late 19th century who proposed the Neogrammarian hypothesis of the regularity of sound change. According to this hypothesis, a diachronic sound change affects simultaneously all words in which its environment is met, without exception. Verner's law is a famous example of the Neogrammarian hypothesis, as it resolved an apparent exception to Grimm's law. The Neogrammarian hypothesis was the first hypothesis of sound change to attempt to follow the principle of falsifiability according to scientific method. Subsequent researchers have questioned this hypothesis from two perspectives. First, adherents of lexical diffusion (where a sound change affects only a few words at first and then gradually spreads to other words) believe that some words undergo changes before others. Second, some believe that it is possible for sound changes to observe grammatical conditioning. Nonetheless, both of these challenges to exceptionlessness remain controversial, and many investigators continue to adhere to the Neogrammarian doctrine.